The present invention relates to a multi-gradation recording method according to a thermal transfer recording system and a thermal transfer recording medium to be used for the method. More particularly, the present invention relates to a multi-gradation recording method for forming a color image with multi-gradation, wherein gradation recording is carried out by transferring an ink layer to an image receiving body using a thermal printer and a thermal transfer recording medium to be employed for the multi-gradation recording method.
Recording according to a thermal transfer recording method using a thermal printer is a dot recording and a gradation with dots can be expressed in terms of, for example, the density of dots (number of dots per unit area) or the size of dots utilizing a halftone dot effect. As the gradation expression utilizing a halftone dot effect, the following are available: (1) a density gradation method wherein the thickness of ink dots transferred from an ink layer is changed while keeping the area of each dot constant; (2) an area gradation method (so-called xe2x80x9cdither methodxe2x80x9d) wherein one pixel is composed of a plurality of dots (matrix) arranged in a zigzag pattern and the number of dots in one pixel is changed while keeping the thickness of transferred ink dots constant (i.e., binary recording); (3) an area gradation method (so-called xe2x80x9cvariable dot methodxe2x80x9d) wherein the area of dots is changed while keeping the thickness of transferred ink dots constant; and the like.
As the density gradation method (1), JP, A, 58-185294 discloses a method for expressing a gradation of a middle tone using a thermal transfer recording medium in which a plurality of ink layers with different reflective optical densities are stacked on a substrate such that the reflective optical densities and melting points of the ink layers are lowered as ink layers are more removed from the substrate. However, since the binders of the plurality of ink layers are similar types of materials such as waxes, even if transfer of only the uppermost pale color layer is tried, the layer is mixed with an ink layer thereunder. This results in a problem in that transfer of portions of the ink layer thereunder occurs together. Even if a significant difference in melting point between the respective ink layers is provided, it is difficult to control the energy such that a desired ink layer is transferred since the inks are composed of similar types of materials. Hence, a middle tone including a satisfactory gradation in a low density region cannot be reproduced. Further, JP, A, 58-205798 proposes a method for obtaining a high quality gradation expression ranging from a low density to a high density by providing a plurality of thermal transfer recording media with the same color hue and different reflective optical densities and by using a thermal transfer recording media for low density to form dots in a low density region and a thermal transfer recording media for high density to form dots in a high density region. However, in this method, it is necessary to frequently change the thermal transfer recording media and in practice, the method can provide at most about two gradations and cannot provide a satisfactory gradation performance.
In the area gradation method (2) (dither method) changing the density of dots (the number of dots in a matrix), the heating resistors of a thermal head are restricted in size and cannot be made small to a far extent, thereby causing a problem in that the resolution is deteriorated when the area of one pixel is made larger to obtain an image with a large number of gradations.
Recently, the area gradation method (3) changing the area of dots has rapidly spread as an efficient method for obtaining a high-definition full-color image. As thermal transfer recording media suitable for a multi-gradation recording method utilizing only the area gradation method (3), various types of recording media have been proposed (see JP, A, 7-117359 and JP, A, 10-272847). JP, A, 7-117359 discloses that multi-gradation recording is performed using a thermal transfer recording medium having a thinner heat-sensitive ink layer than a conventional one. According to this method, an excellent gradation image can be obtained in a high density/middle density region. However this method has a problem in that, since the reflective optical density per unit area of dots with a size in the neighborhood of that of the minimum transferred dot is not that much different from the reflective optical density per unit area of other bigger transferred dots, the resulting image has a grainy appearance (visually rough impression) due to the small transferred dots.
This conventional technique will be described with reference to the accompanying drawings. FIG. 3 is a partial cross-sectional view showing a thermal transfer recording medium employed for the conventional technique. The thermal transfer recording medium comprises a substrate 11 and a release layer 12 and a heat-sensitive ink layer 13 stacked in this order on one side of the substrate 11. FIG. 4 is a schematic view showing a gradation recording performed on an image receiving body 15 using the thermal transfer recording medium by a variable dot method. In FIG. 4, reference numeral 16 denotes ink dots transferred while being changed in area and excellent gradation performance is obtained in a middle density/high density region MH. However, even by the variable dot method, the size of the minimum dot 17 transferred in binary recording is limited and dots 18 with smaller sizes than the minimum dot 17 cannot be transferred. Moreover, the reflective optical density per unit area of dots with sizes in the neighborhood of the size of the minimum transferred dot is not that different from the reflective optical density per unit area of other bigger transferred dots. Therefore, the gradation performance is not satisfactory in a low density region L and the resulting image gives grainy appearance due to the transferred dots with sizes in the neighborhood of the size of the minimum transferred dot.
It is an object of the present invention to provide a multi-gradation recording method capable of carrying out gradation recording with excellent gradation performance, especially in a low density region, without giving grainy appearance (visually rough impression), thereby obtaining a high-quality full-color image.
Another object of the present invention is to provide a thermal transfer recording medium suitable for the foregoing multi-gradation recording method.
These and other objects of the present invention will become apparent from the description hereinafter.
The present invention provides the following multi-gradation recording methods and thermal transfer recording media used in the methods:
(1) A multi-gradation recording method for carrying out multi-gradation recording according to a thermal transfer recording method, comprising selectively heating a thermal transfer recording medium from the rear side thereof with a thermal head to transfer an ink layer thereon on a dot basis to an image receiving body, the multi-gradation recording method comprising the steps of:
providing a thermal transfer recording medium comprising a substrate, a release layer, a first ink layer and a second ink layer provided in this order on the substrate, and
carrying out a gradation recording using the thermal transfer recording medium,
the step of carrying out a gradation recording comprising:
(a) in the case of recording in a middle density/high density region, carrying out a gradation expression based on an area gradation expression by transferring the first ink layer and the second ink layer together, and
(b) in the case of recording in a low density region, carrying out a gradation expression by transferring only the second ink layer by causing a cohesive failure in the second ink layer, thereby changing the area of transferred ink per one dot and the thickness of transferred ink dots.
(2) The multi-gradation recording method of the above (1), wherein the release layer comprises a wax as a main component by weight, each of the first ink layer and the second ink layer comprises a coloring agent and a binder, each of the binder in the first ink layer and the binder of the second ink layer comprises a thermoplastic resin as a main component by weight, the thermoplastic resin as the main component of the binder of the first ink layer and the thermoplastic resin as the main component of the binder of the second ink layer are substantially incompatible with each other, the release layer has a melting point of 60xc2x0 to 120xc2x0 C. and a heat of fusion of 100 mJ/mg or more, the binder of the second ink layer has a softening point of 50xc2x0 to 90xc2x0 C. and a tensile strength (JIS K 6760 and JIS K 7113) of smaller than 200 kg/cm2, and the binder of the first ink layer has a softening point higher than the softening point of the binder of the second ink layer and a tensile strength (JIS K 6760 and JIS K 7113) of not smaller than 200 kg/cm2.
(3) The multi-gradation recording method of the above (2), wherein the binder of the second ink layer comprises at least one member of an ethylene-vinyl acetate copolymer and an ethylene-ethyl acrylate copolymer as a main component by weight.
(4) The multi-gradation recording method of the above (2) or (3), wherein the content (g/m2) of the coloring agent per unit area in the first ink layer is higher than the content (g/m2) of the coloring agent per unit area in the second ink layer.
(5) The multi-gradation recording method of any one of the above (2) to (4), wherein the thermal transfer recording medium has a total thickness including the thickness of the substrate of not more than 5.0 xcexcm.
(6) A thermal transfer recording medium used in a multi-gradation recording method of the above (1), comprising a substrate, a release layer, a first ink layer and a second ink layer provided in this order on the substrate,
wherein the release layer comprises a wax as a main component by weight, each of the first ink layer and the second ink layer comprises a coloring agent and a binder, each of the binder in the first ink layer and the binder of the second ink layer comprises a thermoplastic resin as a main component by weight, the thermoplastic resin as the main component of the binder of the first ink layer and the thermoplastic resin as the main component of the binder of the second ink layer are substantially incompatible with each other, the release layer has a melting point of 60xc2x0 to 120xc2x0 C. and a heat of fusion of 100 mJ/mg or more, the binder of the second ink layer has a softening point of 50xc2x0 to 90xc2x0 C. and a tensile strength (JIS K 6760 and JIS K 7113) of smaller than 200 kg/cm2, and the binder of the first ink layer has a softening point higher than the softening point of the binder of the second ink layer and a tensile strength (JIS K 6760 and JIS K 7113) of not smaller than 200 kg/cm2.
(7) The thermal transfer recording medium of the above (6), wherein the binder of the second ink layer comprises at least one member of an ethylene-vinyl acetate copolymer and an ethylene-ethyl acrylate copolymer as a main component by weight.
(8) The thermal transfer recording medium of the above (6) or (7), wherein the content (g/m2) of the coloring agent per unit area in the first ink layer is higher than the content (g/m2) of the coloring agent per unit area in the second ink layer.
(9) The thermal transfer recording medium of any one of the above (6) to (8), which has a total thickness including the thickness of the substrate of not more than 5.0 xcexcm.